Human infants mount relatively poor quality antibody responses of low titer to primary viral infections such as rotavirus. It has been speculated that the B cell repertoire of infants is limited because of residual fetal-like constrained VH usage, and that this is a principal mechanism underlying the poor antibody response of infants to foreign antigens early in life. If this is so, then attempts to vaccinate infants at very young ages with novel antigens complexed with adjuvants that enhance antigen presentation or T cell help are unlikely to be effective. Therefore it is critical to determine if the B cell repertoire in infants is functionally mature. The central hypothesis of this proposal is that infant B cell repertoires are sufficiently diversified for mature virus-specific responses, but lack somatic mutations. Three specific aims are proposed: SPECIFIC AIM 1. To determine the genetic basis for binding of infant and adult RV-specific Abs, using detailed analysis of variable gene regions induced by infection. We will identify the immunodominant variable genes used to make RV-specific Abs, and will determine whether infants use the same Ab variable genes as adults. SPECIFIC AIM 2. To identify the structural determinants of optimal binding of anti-RV Abs. We will test the hypothesis that infant Abs using unmutated immunodominant VH segments in the RV Ab response (such as VH1-46) bind to RV with low affinity, but naturally occurring mutated adult VH1-46 Abs bind RV with high affinity. SPECIFIC AIM 3. To test the hypothesis that amino acid residues in mutated adult Abs that contribute to increased affinity of interaction of VP7 Abs confer a higher activity for virus neutralization. This work addresses whether increases in affinity at the biochemical level of Ag-Ab interaction increase functional activity (potency) of Abs. This work will demonstrate that infant B cell repertoires used to make specific antiviral responses, in contrast to much previous speculation, are much closer in character to adult repertoires than fetal repertoires. [unreadable] [unreadable]